Method and device for producing cold and liquefying gases

ABSTRACT

A process of producing refrigeration and liquefying gasses in which the gas is compressed and refrigerated, to have specific enthalpy, before expansion through a jet ejector, that corresponds to the enthalpy associated with the constant enthalpy line substantially tangent to the boundary line of the liquid-vapor region in the temperature-enthrophy diagram of the medium. The gas after passing through the ejector is collected in a high-pressure reservoir which communicates through a throttling valve with the vapor space in the low pressure reservoir.

1971 J. A. VAN DEN BORN ETAL 3,557,566

METHOD AND DEVICE FOR PRODUCING COLD AND LIQUEFYING GASES Filed July 10, 1968 '2 Sheets-$heet 1 COLD GAS REFRIGERATORS HEATER/ 1 j 3 22 W MINIMUM PRESSURE 5 24 2 3 RESEVOIR EJECTOR15 I 55213":

JACOBUS A.VAN DEN BORN INVENTOR JAN HAISMA FERDINAND J.M KROON AGENT Jam. 26, '1971 J. A. VAN DEN BORN ET L METHOD AND DEVICE FOR PRODUOING COLD AND LIQUEFYING GASES Filed July 10, 1968 2 Sheets-Sheet 2 JACOBUS A.VAN DEN BORN INVENTOR) JAN S FERDINAND J;.-M.KROON ZMA GENT "United States Patent 01 ice US. Cl. 629 Claims ABSTRACT OF THE DISCLOSURE A process of producing refrigeration and liquefying gasses in which the gas is compressed and refrigerated, to have specific enthalpy, before expansion through a jet ejector, that corresponds to the enthalpy associated with the constant enthalpy line substantially tangent to the boundary line of the liquid-vapour region in the temperature-enthrophy diagram of the medium. The gas after passing through the ejector is collected in a high-pressure reservoir which communicates through a throttling valve with the vapor space in the low pressure reservoir.

The invention relates to a method for producing cold and for liquefying gases, in which a high-pressure medium is cooled below its inversion temperature associated with the relevant pressure and is then supplied to the jet nozzle of at least one ejector; the suction side of the ejector is connected to a minimum pressure reservoir which then communicates through a choking cock to a further reservoir, an outlet duct of the further reservoir communicates with the ejector outlet. The invention furthermore relates to a device suitable for carrying out this method.

In the scope of the present application the term ejector is to denote a device in which the potential energy of a high-pressure (primary) medium is converted wholly or partly into kinetic energy, which is utilized at least partly for raising the pressure of a secondary medium.

In the method and the device according to the invention the energy of the high-pressure medium supplied to the ejector is utilized at least partly for drawing off the vapour from the minimum pressure reservoir and to raise this pressure to the value prevailing in the outlet. Consequently, in the minimum pressure reservoir a lower pressure and hence a lower temperature will prevail than in the outlet. The cold is therefore supplied at a lower temperature than that corresponding to the pressure of the medium in the outlet. This has the advantage that in the heat exchangers in which the high-pressure medium and the expanded medium are in relative thermal contact, an advantageous pressure ratio is obtained. In closed systems,

in which the high-pressure medium is supplied by a compressor and the outlet communicates with the suction side of the compressor, the compression ratio may be comparatively low. Owing to the aforesaid properties the method and the device according to the invention are particularly suitable for producing cold at very low temperatures, with which very low vapour pressures are associated. A particular phenomenon in this case is that in the ejector, not only a reduction of pressure of the primary medium owing to the Joule-Kelvin effect, but also a compression of the secondary medium are obtained. The need for cold at temperatures below 4 K. is constantly increasing, particularly for cooling devices such as supraconductive coils and cryogenic computer or storage elements of electronic computers. With these temperatures are associated, as stated above, very low vapour 3,557,566 Patented Jan. 26, 1971 pressures above the liquid in the minimum pressure reservoir, in which the object to be cooled is arranged. If, for example, cold is to be supplied at 1 K., a vapour pressure of helimum in the minimum pressure reservoir of 0.12 mm. Hg is involved. In order to maintain these loW pressures in the minimum pressure reservoir, an extremely effective suction action of the ejector is required.

The suction pressure obtainable at a minimum with an ejector is determined by the speed with which the medium leaves the jet nozzle. Therefore, an ultrasonic structure of the jet nozzle is necessary for attaining these low suction pressures. The invention has for its object to provide a method and a device of the kind set forth, in which owing to the lower suction pressure of the ejector obtainable cold can be supplied at lower temperatures than has hitherto been possible.

The method according to the invention is characterized in that the temperature and the pressure of the cooled high-pressure medium prior to entering the jet nozzle are chosen so that the enthalpy of the medium corresponds with the enthalpy associated with the line of constant enthalpy which is at least approximately tangential to the boundary line of the liquid-vapour region in the temperature-entropy diagram of the medium concerned.

It has surprisingly been found that the said adjustment of the enthalpy of the medium supplied to the jet nozzle results in that the suction pressure attainable at a minimum drops by a factor 10. This may be accounted for by the fact that the enthalpy of the supplied medium lies on the line of constant enthalpy which is tangential to the boundary line of the liquid-vapour region in the temperature-entropy diagram so that after choking in the ejector the medium is still in the vapour phase. This prevents the formation of liquid droplets in the diffusor. These liquid droplets have a disturbing effect on the flow process.

According to a further method embodying the invention the temperature and the pressure of the high-pressure medium, prior to entering the jet nozzle, are chosen so that the entropy of this medium is lower than the entropy associated with its critical point.

The invention furtermore relates to a device suitable for carrying out said method, which device comprises a high-pressure medium supply communicating through one or more heat exchangers, in which this medium is cooled below its inversion temperature associated with said pressure to a jet nozzle of one or more ejectors; the suction side of each ejector communicates with a low-pressure reservoir which communicates through a choking cock and, optionally through a further reservoir, to an outlet duct. The outlet of the ejector also communicates through such further reservoir to the outlet duct. This device is characterized in that after the last heat exchanger the supply duct is in thermal contact with the medium in the ejector outlet or in the further reservoir, while the device comprises further means for adjusting the temperature of the high-pressure medium prior to entry in the jet nozzle so that at the prevailing pressure the temperature of the high-pressure medium prior to entry in the jet nozzle is such that the enthalpy of the medium corresponds substantially with the enthalpy associated with the line of constant enthalpy which is at least approximately tangential to the boundary line of the liquid-vapour region in the temperature-entropy diagram of the medium concerned. By a suitable adjustment of the temperature with a given pressure associated with the flow mass passing through the ejector, so that the enthalpy has a value such that with an isenthalpic choking process no phase transition occurs, a device is provided which is capable of producing, due to the very low suction pressure of the ejector, cold at very low temperatures.

In a further device embodying the invention the means for adjusting the desired temperature of the medium prior to its entry in the jet nozzle, is formed by a controllable heating member, which is in thermal contact with that portion of the supply duct which is located between the jet nozzle and the area where the supply duct is in thermal contact with the ejector outlet or the further reservoir. By means of this controllable heating member the temperature can be regulated so that the enthalpy of the medium invariably corresponds with said value. The heating member may be formed by an electrical incandescent helix or a thermally conductive connection (copper strip) between the cold part of the supply duct and an area of higher temperature.

In a further device embodying the invention the means for the adjustment of the desired temperature is formed by a circulation duct communicating (a) at one end with the supply duct at an area located between (i) the last heat exchanger and (ii) the area where the supply duct is in contact with the ejector outlet or the further reservoir, and (b) at the other end again with the supply duct just before the jet nozzle, said circulation duct being in thermal contact with a heating member by which heat can be supplied to the medium, the device comprising one or more control-cocks by which the portion of the high-pressure medium flow passing through the circulation duct can be regulated.

A further advantageous embodiment is characterized in that there is provided a circulation duct which communicates (a) at one end with the supply duct at an area located in front of the outlet side of said medium leaving the last heat exchanger, viewed in the direction of flow of the high-pressure medium, and (b) at the other end with the supply duct just in front of the jet nozzle, said device comprising one or more control-cocks by which the por tion of the high-pressure medium flow passing through the circulation duct can be regulated. In this way the temperature can be adjusted to the desired value.

The invention will be described more fully with reference to the drawing.

FIG. 1 shows diagrammatically and not to scale a device for producing cold at low temperatures.

FIG. 2 shows diagrammatically and not to scale a temperature-entropy diagram.

Referring'to FIG. 1 reference numeral 1 designates a compressor. The outlet 2 of this compressor communicates with a heat exchanger 3, where the compressed high-pressure medium is in thermal contact with expanded lowpressure medium flowing to the inlet 4. Then the compressed medium in the heat exchanger 5 is in thermal contact with a cold-gas refrigerator 6, shown diagrammatically. The high-pressure medium is then in the heat exchanger 7 in thermal contact with expanded medium. In the heat exchanger 8 the medium is further cooled by a cold-gas refrigerator 9, after which in the heat exchanger 10 the medium is again in thermal contact with expanded medium. The outlet of the last heat exchanger 10 communicates with a stream dividing cock 11. With this dividing cock communicate two ducts 12 and 13, which joint at the area 14, where they communicate in common with the jet nozzle 15 of the ejector 16. The diffusor 17 of the ejector 16 communicates with a duct 18, which opens out in a reservoir 19. The vapour space of the reservoir 19 communicates through a duct 20 and the low-pressure sides of the heat exchangers 10, 7 and 3 with the inlet valve 4 of the compressor 1. The reservoir 19 communicates through the duct 21 including a choking cock 22 with the minimum pressure reservoir 23. The vapour space of the minimum pressure reservoir 23 communicates openly with the suction side 24 of the ejector 16. The duct 13 is in thermal contact at 25 with the medium in the reservoir 19, and the duct 12 is in thermal contact with the heating member 26.

The operation of this device is as follows: Helium is compressed in the compressor 1 to about atm. and then cooled in the heat exchangers 3, 5, 7, 8, and 10. In the dividing cock 11 the supplied How of helium is divided into a portion passing through the duct 13 in contact with liquid helium under atmospheric pressure, and a portion passing through the duct 12 in contact with the heating member 26. The two portions of the stream are remixed at 14. The cock 11 divides the stream so that at the area 14 the medium has an enthalpy corresponding with the enthalpy associated with the isenthalpy tangential to the boundary line of the liquid-vapour region. In FIG. 2 the boundary line of the liquid-vapour region is indicated by 30 and the tangential isenthalpy by 31 in the T5. diagram. If the pressure of the medium is 30 atm. the temperature at the area 14 has to be T In this case the point 33', determined by 12:30 atm. and T is just located on the isenthalpy curve 31, which is tangential to the liquid-vapour region. If the pressure is lower, for example, 20 atm., because a smaller circulating mass stream is desired, the dividing cock 11 has to be adjusted so that a larger mass stream passes through the duct 12. The temperature at the area 14 will thus increase until the enthalpy corresponds with the enthalpy of point 34'. It is then ensured that the initial conditions are such that the entropy value of the medium prior to its entry in the ejector is lower than the entropy value associated with the critical point 35'. In this way a very satisfactory suction effect is always obtained.

It has been found that by this adjustment of the enthalpy the suction pressure may be a factor 10 lower than in devices in which the medium stream is cooled prior to entry in the jet nozzle to the utmost, that is to say to about 4 K. In the device shown in FIG. 1 a pressure of 10 mm. Hg can be maintained in the reservoir 23, which corresponds to a temperature of K. The helium supplied to the jet nozzle 15 expands therein and at the end of the jet nozzle, owing to its ultrasonic shape, the

.- helium will have a very high speed and a low pressure. In

the diffusor 17 the pressure of the medium together with the vapour sucked out of the reservoir 23 increases to about 1 atmosphere and then flows through the duct 18 to the reservoir 19. From the reservoir 19 vapour flows through the outlet 20 and said heat exchangers to the inlet of the compressor. A further portion of the medium of the reservoir 19 is supplied via the choking cock 22, where the pressure is reduced, to the reservoir 23. In this way a device is obtained which permits of obtaining cold at very low temperatures in the reservoir 23.

FIG. 3 shows schematically a device corresponding largely with that of FIG. 1. The adjustment of the desired temperature at the area 14 is obtained in this case by the whole high-pressure helium stream being in thermal contact in the heat exchanger 25 with the liquid helium in the reservoir 19, after which the whole helium stream is in thermal contact with a controllable heating member 30. By the controllable heating device, a quantity of heat can be supplied to the high-pressure helium stream, and the temperature at the area 14 is such that the enthalpy again corresponds with the value of the isenthalpy curve 31 of the diagram of FIG. 2 for the prevailing pressure.

F IG. 4 shows schematically an arrangement corresponding with the arrangements of FIGS. 1 and 3. In this arrangement there is provided a circulation duct 32, which communicates at one end at the area 33 through a controlcock 34 with the high-pressure helium supply duct. The area 33 is located, viewed in the direction of fiow of the high-pressure medium, in front of the outlet side of the last heat exchanger 10. The circulation duct 32 communicates at the area 14 with the high-pressure medium supply duct.

By regulating the cock 34 part of the high-pressure helium stream flows via the duct 32 to the area 14. This portion of the helium stream has a higher temperature than the other portion of the helium stream which has completely passed through the heat exchanger 10 and is further cooled in the heat exchanger 25. By an appropriate adjustment of the cock 34 such a quantity will pass through the circulation duct 32 that at the area 14 the desired temperature is again attained.

It will be obvious that the connection of the circulation duct 32 at the area 33 with the supply duct is chosen only by Way of example. If desired, the connection may be established farther upstream for example at the area 40 or farther downstream, for example, at the area 41.

What is claimed is:

1. In a method of producing cold and liquifying gases, including cooling a high-pressure medium to below its inversion temperature associated with the relevant pressure, and then expanding said medium through a jet nozzle of an ejector the suction side of which communicates with a minimum pressure reservoir which in turn communicates through a throttling valve to a further reservoir communicating in turn with the ejector outlet, comprising the further steps of controlling the temperature and the pressure of the cooled high-pressure medium prior to its entry in the jet nozzle, such that the enthalpy of the medium approximately corresponds to the enthalpy associated with the constant enthalpy line which is at least substantially tangent to the boundary line of the liquid-vapour region in the temperature-entropy diagram of the medium concerned.

'2. A method according to claim 1 comprising the further step, prior to entry of the medium into the jet nozzle,

. of separating the flow of the medium into two streams,

cooling one stream to a lower temperature than the other, subsequently joining the stream with their temperatures controlled as required to achieve the appropriate temperature for said selected enthalpy.

3. A method according to claim 1 comprising the further steps: prior to entry of the medium into the jet nozzle, separating the flow of the medium into two streams and heating one stream to a temperature higher than the other, subsequetnly joining the streams, with their temperatures controlled as required to achieve the appropriate temperature for said selected enthalpy.

4. A method according to claim 1 comprising the further step of controlling the temperature and the pressure of the high-pressure medium prior to its entry into the jet nozzle, such that the entropy of this medium is lower than the entropy associated with the critical point of said medium.

5. In a device for refrigerating and liquifying gases including a high-pressure medium supply, at least one refrigeration means for cooling the medium to below its inversion temperature, an ejector nozzle including (i) an inlet for receiving the medium from the heat exchanger, (ii) an outlet, and (iii) a suction side for expansion of the medium, a low-pressure reservoir communicating with said suction side, a throttling valve, and a further reservoir communicating (i) with the low-pressure reservoir via the throttling valve, (ii) with the nozzle outlet, and (iii) with said high-pressure source, regulating means for adjusting the temperature of the highpressure medium prior to its entry in the jet nozzle, so that at the prevailing pressure, the temperature of said medium is such that its enthalpy corresponds substantially to the enthalpy associated with the line of constant enthalpy which is at least substantially tangential to the boundary line of the liquid-vapour region in the tempera ture-entropy diagram of the medium concerned.

6. A device according to claim 5 wherein said regulating means controls the temperature and the pressure of the high-pressure medium prior to its entry in the jet nozzle such that the entropy of this medium is lower than the entropy associated with the critical point of said meduim.

7. A device according to claim 5 further comprising a supply duct for transporting medium from the source through the refrigerator means and through the high-pressure reservoir for further cooling the medium to the nozzle inlet, the regulating means providing heat to said supply duct and medium therein immediately prior to its entry of the nozzle.

8. A device according to claim 5 further comprising a supply duct for transporting medium from the source through the refrigeration means and through the highpressure reservoir for futher cooling the medium, and an auxiliary duct inter-connecting said nozzle and supply duct while by-passing the high-pressure reservoir, and valve means for selectively flowing medium from the medium supply through the supply and auxiliary ducts, whereby some of the medium from the supply duct is transportable via the auxiliary duct to the nozzle in a less-cooled state.

9. A device according to claim 8 further comprising means for heating the medium in the auxiliary duct.

10. A device according to claim 8 wherein said refriger- McMahon, H. 0.: Recent Developments in Gas Cryogenics, Cryogenics, December 1960, pp. -70.

NORMAN YUDKOFF, Primary Examiner A. F. PURCELL, Assistant Examiner US. Cl. X.R. 62-268, 467 

